Layered double hydroxide (LDH) is layered compounds having cationic double hydroxide layers and charge-balancing interlayer anions (negative ions), and are also referred to as hydrotalcite-like substances.
Generally, if clay minerals such as montmorillonite contact water or organic solvent, solvent molecules enter between layers, increasing interlayer distance and thus causing swelling and gelation. It is known that if swelling continues, each layer is separated and delaminated from each other, and eventually turns to nanosheets, and transparent colloidal solution, namely sol, is obtained.
It has recently been found that with layered inorganic compounds other than clay minerals such as titanic acid compounds also, by introducing specific organic ions between layers, the layers are delaminated from each other in water, turning into nanosheets, and studies have been conducted to form nano-multilayered structures by layer-by-layer self assembly process wherein a cationic sheet and an anionic sheet are deposited alternately.
Double hydroxide nanosheets obtained from LDH are attracting attention for the following three reasons:    (1) Many inorganic nanosheets exhibit anionic property, and there are only a few cationic nanosheets. LDH layers are cationic, and cationic nanosheets can be laminated alternately with anionic nanosheets (Non-patent Literature 1 and Patent Literature 1),    (2) Various magnetic or electrically conductive divalent and trivalent metal ions can be introduced into LDH layers, which increases the possibility of material design (Non-patent Literatures 2 to 4), and    (3) Synthesis of LDH itself is relatively easy.
Focusing on such advantages of LDH, attempts have been made to produce cationic nanosheets by allowing LDH to swell, causing delamination.
Recent finding that LDH could be delaminated by formamide (HCONH2) has paved the way for producing LDH nanosheets (Non-patent Literatures 1, 5, 6, 7 and Patent Literatures 1, 2). However, since formamide hardly evaporates and is toxic to humans, and alternate lamination is performed not in an organic solvent such as formamide but in water in many cases, delamination and generation of nanosheets in water have been desired.
Delamination of LDH in water has been achieved by introducing carboxylate anions, which are organic anions, between layers. For example, it is reported that LDH delamination containing lactate anions (CH3—CH(OH)—COO−) between layers (Non-patent Literature 8 and Patent Literature 3) and LDH delamination containing magnesium acrylate salt and/or magnesium acetate salt between layers (Patent Literatures 4, 5). However, all of the above have adopted “the reconstruction process”, where LDH structurally altered by heat treatment performed at temperatures close to as high as 600° C. are immersed in a salt solution containing these carboxylate anions to cause reaction to occur. Consequently, the crystallinity of the obtained LDH is low, crystalline form is instable, and crystal size is as small as approximately several nanometers. It was therefore difficult to obtain large high-quality LDH nanosheets. Furthermore, with the above reconstruction process, the composition of constituent metal ions may vary (Non-patent Literature 9). In addition, as described in the above Non-patent Literature 8, since nearly 50% of the interlayer anions are carbonate ions, reflection peak in X-ray diffraction is broad, and crystallinity is low. Furthermore, the Mg/Al molar ratio of the LDH to be used as a starting material is 3 in many cases, and there are only a small number of verification examples of LDH having Mg/Al molar ratio of 2, where the layer electric charge density is high and thus delamination is considered to be difficult, or LDH of metal ions other than Mg and Al LDH delamination using water has thus lacked versatility.
Inventors have succeeded in the synthesis of water-swelling LDH, which includes acetate anions between the layers by an anion exchange method to replace the anion in LDH interlayer. Above synthesis of water-swelling LDH has been filed as patent application (Patent Literature 6, Patent Literature 7 and Non-patent Literature 11). The obtained acetate anion-type LDH is superior to other water-swelling LDH from the viewpoints of purity and delamination property. However, if stored in open state in the air, it reacts with carbon dioxide in the air, and turns into a carbonate ion-type LDH by discharging acetate anions. It is therefore necessary to store it in a tightly-capped container, and its stability is not perfect (Patent Literature 6, Non-patent Literature 10). Odor resulting from discharge of acetic acid is another problem.
Attempts have been made to synthesize water-swelling LDH by introducing organic carboxylate anions between layers, and each of such attempts has found respectively distinguishing water-swelling LDH. However, as described above, high-quality water-swelling LDH satisfying all the conditions including high crystallinity, high purity, high stability, versatility, scentlessness, and non-toxicity have not been found yet.